Field of the Invention
The present invention relates to a combined fuel cell and boiler system, and more particularly to a combined fuel cell and boiler system, in which thermal efficiency of a boiler may be improved by using an exhaust gas from a fuel cell.
Description of the Related Art
In general, most of energies used by humans have been obtained from fossil fuels. However, the use of the fossil fuels has significantly adverse effects on environment, such as air pollution, acid rain, global warming, and also brings about a problem of low energy efficiency.
To solve the problems caused by using the fossil fuels, fuel cell systems have been recently developed. The fuel cells are different from typical secondary batteries in that the fuel cells have a structure in which electricity is generated by supplying fuels, such as hydrogen gas or hydrocarbon, to an anode and supplying oxygen to a cathode.
That is, fuel cells are referred to as “cells”, however in reality, may be regarded as power generators for generating electricity. Basically, the fuel cells employ a method in which fuels are not combusted but an electro-chemical reaction between hydrogen and oxygen is triggered, and an energy difference before and after the reaction is converted to electrical energy.
The fuel cells do not generate gases, such as NOx, SOx, which cause environmental pollution, and are noiseless and vibrationless systems, so that the fuel cells may be considered a clean power-generation system having the thermal efficiency of 80% or more by adding up an electricity generation amount and a heat collecting amount.
The reaction between hydrogen and oxygen in a fuel cell is an exothermic reaction, and thus generates heat. Phosphoric acid is mainly used as an electrolyte in the fuel cell, and such a phosphoric acid fuel cell is known to have an operation temperature of about 200° C. This temperature is a maximum allowable temperature for a phosphoric acid electrolyte. The reaction between hydrogen and oxygen is most actively performed at about 200° C., which is a reaction temperature of a fuel cell; however, heat is generated by the exothermic reaction between hydrogen and oxygen and thus the reaction between hydrogen and oxygen is not readily performed, thereby leading to a decrease in efficiency. Accordingly, a cooling structure for cooling the fuel cell is essentially required.
Also, as another example of a fuel cell, there is a molten carbonate type fuel cell in which a mixture of lithium carbide and potassium carbide which have a low melting point is used as an electrolyte. An operation temperature of the molten carbonate type fuel cell is about 650° C., and a hot box is installed to maintain the operation temperature.
In order to maintain the operation temperature having a close relation with the efficiency of the fuel cell and to improve power generation efficiency, various fuel cell systems have been proposed.
For example, Korean Patent No. 10-0787244 discloses a structure which includes an air supply unit for supplying oxygen-containing air for effective combustion of raw materials for power generation, and which uses a double intake method of allowing outside air to be introduced for properly lowering the temperature of a hot box so as to generate electrical power using oxygen of the intaken air.
The intaken air is finally discharged to the outside. In this case, the exhaust gas is higher in temperature than outside air, and this may cause a problem in that a white gas is generated when the exhaust gas is discharged.
Also, Korean Patent No. 10-0787244 discloses a structure in which a carbon monoxide remover for removing carbon monoxide is connected to an intake passage in order to supply oxygen-containing air.
However, this structure directly supplies air which is supplied through an outside air intake container, an air intake port, and a filter, to a fuel treatment unit, etc., by using an air supply pressure adjustment unit. Because room-temperature air is directly supplied and used, there may be a phenomenon that an internal temperature of the hot box is lowered. When the temperature of the hot box is thus lowered than a reaction temperature, there is a problem in that power generation efficiency may be lowered.